Question

you wish to detemine the reaction order and rate constant for the following thermal decomposition reaction 2AB2- -> A2 + 2B2 A) what data wolud you collect? B) how would you use this data to determine whether the reaction is zero, first or second order ? C) Describe how you would determin the value of the rate constant?

Answer 1

For given Decomposing Reaction,

2 AB₂ A₂ + 2 B₂

Order of reaction can be determined by experimental methods, In which series of run is carried out and resulted data is tabulated.

For above decomposition reaction,

Tabulate Concentration(M or N) at different time.

Time	Concentration (2 AB2)	Concentration (A2)	Concentration ( 2 B2)
As Increase	Concentration Decreases	Concentration Increses	Concentration Increses

In general form of the reaction:

aA + bB → cC + dD

Plot C _AB2 vs Time Curve

For Four to five values of time Find instantaneous value of rate of reaction,

It can be found by slope of C vs. T curve at any time when we want to find instantaneous rate of reaction (-r_AB2)

   (R = reactant, P= Product Concentraton)

For at different value of time obtain value of (-r_AB2)

Time	-rAB2

From abobe table To find Order of reaction there is many mathods such as differential method Integral method.

Then start assumption from zero order and then first order and keep trial runs,

Assume a rate equation of the form
-r_A = -dC_A/dt = k C_Aⁿ

If we get a straight line passing through the origin then the rate equation is consistent with the data.(i.e. the assumed rate equation is correct, i.e. the guessed order of reaction is correct).

if we do not get a straight line then another equation should be tested.

If Assumed rate equation being consistent with data then we may proceed further to find value of k(Rate constant).

We have,
(-r_AB2) = -(dC_AB2/dt) = k C_AB2ⁿ
∴ ln(-r_AB2) = ln k + n ln C_AB2

Plot a graph of ln(-rA) v/s ln CA. It yields a straight line with a slope equal to n(i.e., order of reaction with respect to A) and the intercept gives the value of k.